Wide range resonator



June 26, 1956 H. A. FINKE EI'AL WIDE RANGE RESONATOR Filed Aug. 22. 1951 INVENTORS HEREERTAF/NKE JOHNEEBERT BY aw fl ATTORNEY- PLUNGZR United States Patent WIDE RANGE RESONATOR Herbert A. Finke, Brooklyn, and John E. Ebert, Lynbrook,

N. Y., assignors to Polytechnic Research and Development Company Incorporated, Brooklyn, N. Y., a corporation of New York Application August 22, 1951, Serial No. 243,078

7 Claims. (Cl. 250-36) This invention relates to an electric wave resonator which may be varied in resonant frequency.

Our improved resonator may be used as a variable frequency oscillator, as a cavity type frequency meter, as a tuned load for a broad band amplifier, or in other situations where a resonator is required to cover a resonant frequency which may be varied over a broad range. For the purpose of illustration, the invention will be illustrated and described as applied to a variable frequency oscillator.

A broad object of the invention is to design a resonator which may be variably tuned over a Wide range of frequencies within the very high and ultra high frequency bands. For example, our improved resonator can be designed to cover the range from 20 megacycles per second to 1,000 megacycles per second, which provides a tuning ratio of 50 to 1.

Another object is to design a coaxial cavity resonator which has a short physical length for the range of frequencies covered by the resonator. This object is attained by forming the center conductor of the resonator as a helix over the low frequency range.

Still another object is to design a resonator having a desirable frequency-displacement characteristic.

A further object is to provide a novel filter or choke in the connections for energizing the filament of the oscillator tube.

Figure l of the accompanying drawing is a diagrammatic representation of one embodiment of the invention applied to a variable frequency oscillator.

Figure 2 of the drawing is a curve showing the frequency-displacement characteristic of the oscillator of Figure 1.

Figure 3 shows a modified form of resonator.

Referring to Figure 1, the resonator is formed of a length of coaxial cable having a tubular outer conductor 1, and a center conductor 2 which also is of tubular form. Conductors 1 and 2 are supported so that they are insulated from each other at one end of the cable section, and the length of the cavity in the resonator is varied by means of a short-circuiting slider 3 which surrounds cerrter conductor 2 and is provided with two sets of resilient contact fingers for making good electrical contact with the outer and inner conductors respectively. As will be understood, the resonant frequency of the cavity decreases as the slider 3 moves away from the open end of the resonator. For the purpose of shortening the physical length of the resonator for the frequency range to be covered, the low frequency end of the center conductor 2 is constructed as a helix, and this may be accomplished simply by cutting a helical slot 2a in the tubular conductor 2. As shown in Figure l, the pitch of the slot 2a increases from the low frequency end of the resonator towards the high frequency end, and the slot 2a is terminated short of the free end of the center conductor so that a solid section of tubular conductor 2 is used to cover the high frequency portion of the tuning range.

The term pitc refers to the distance between turns Patented June 26, 1956 of the helix along the axis of the resonator, and it will be observed that the number of turns per inch along the axis increases progressively towards the low frequency end of the resonator.

It is preferred to form the helical gap 2a of a width of only a fraction of a millimeter so as to insure good metallic contact between the helical conductor and the maximum number of fingers of the slider 3.

Over the high frequency range of movement of the slider, say from 500mc./s. to 1000 mc./s., the slider exposes only the solid tubular section of the inner conductor 2, and the cavity resonates in the normal coaxial trans-v mission modes. In the low frequency range of tuning, the tuning slider or plunger exposes a center conductor which consists essentially of a coil. The inductive loading of the center conductor reduces the axial velocity of propagation. The result is that the antiresonant frequency of the cavity is low by comparison with the short physical length of the resonator.

In using the resonator in an o-scillaation generator, the free end of theinner conductor 2 is connected to the plate electrode of a suitable electron tube 4, such as a 6P4 triode. The cathode heater filament is energized from a suitable source of current, and the leads to the filament include a two-Wire choke coil 5 positioned within a metallic housing 6 which is grounded and also connected with the outer tubular conductor 1 of the resonator. The filament leads are introduced into the casing 6 by means of conventional button condensers shown in the drawing. Each lead includes one conductor of the twowire solenoid 5.

The negative terminal (B) of a suitable source of plate current is connected to one filament lead through an adjustable biasing resistor 7, and the grid of tube 4 is connected through a high resistance element 8 to the negative end of resistor 7. Condenser 9 represents the capacity provided by a button condenser through which the grid lead of the tube 4 is introduced into the casing 6, the tube 4 being mounted on this casing. This button condenser, for the range of frequency indicated above, would have a capacity value of approximately 2,000 micro-microfarads.

From Figure 1 it will be seen that the coaxial resonator constitutes a tank circuit connected between the plate and grid elements of tube 4. In order for this arrangement to oscillate over a wide range of frequency it is necessary for the direct current return path between the grid and the cathode to have a high impedance value over the entire frequency range. This is provided by the two-wire choke coil 5 arranged in the supply leads of the cathode heater filament.

If the coil is designed so that its antiresonant frequency is in the neighborhood of, or below, the lowest frequency of the resonator, it will provide the necessary high impedance in the grid-cathode return path throughout the tuning range. The coil, preferably, should be a single.- layer, air-cored coil.

Instead of having the resonant chamber formed of an open-ended coaxial section, the chamber may be completely closed. For example, as shown in Figure 3, the outer tubular conductor 1 may be extended beyond the end of the inner tubular conductor 2, and the end of the outer conductor 1 may be closed by a conducting wall 1a spaced from the insulated or free end of the inner conductor 2. If desired, the insulated end of the inner tubular conductor may also be closed by a conducting wall arranged parallel with the end wall In. The connection from the insulated end of the inner conductor 2 to the anode of oscillator tube 4 may be brought out of the resonator through an opening in the side wall of the outer conductor 1, as shown. In all other respects the construction would be the same as shown in Figure 1,

Figure 3 also shows a suitable arrangement for movement of the tuning slider 3 within the resonator. This involves a pair of rods 3a and 35 secured to the slider on opposite sides thereof and passing through suitable bearings in the end wall of the resonator which supports one end of the inner conductor. The outer ends of rods 3a and 3b are joined by a cross-head 3c. The slider may be moved by moving the cross-head 3c manually, or through any suitable operating mechanism.

While the size of the resonator cavity is varied by means of a movable slider having short-circuiting fingers engaging the inner and outer conductors, it will be understood that any other suitable form of movable wall section may be employed for varying the axial length of the resonator. Also, the size of the cavity may remain fixed and helical conductor 2 may be mounted for axial movement into the cavity through an opening therein.

We claim:

1. A broad-band resonator comprising a resonant chamber formed of conducting inner and outer tubular members arranged in concentric relation on a common axis, an annular short-circuiting slider mounted in the space between said inner and outer tubular members and forming one end wall of said chamber, said annular slider being mounted for sliding movement along said axis to vary the length of said chamber, said tubular members being insulated from each other at the other end of said chamber, a portion of said inner tubular member at the insulated end thereof being formed of a solid tubular section extending over a portion of the range of movement of said slider, the remainder of said inner tubular member being formed as a tubular helix.

2. A cavity resonator according to claim 1 wherein the pitch of the turns of the helical section of the inner tubular member progressively decreases towards the lower frequency end of the range of movement of said slider.

3. A resonator according toclaim 1 wherein the outer tubular member extends axially beyond the inner tubular member at the insulated ends thereof, and the end of the outer tubular member is closed by a conducting wall spaced from the insulated end of the inner tubular member.

4. A resonator according to claim 1 wherein the pitch of the turns of the helical section of the inner tubular member progressively decreases towards the low frequency end of the range of movement of said slider, and wherein said outer tubular member extends axially beyond the inner tubular member at the insulated end r range of movement of said slider, and a portion of said center conductor extending over the low frequency section thereof being formed as a tubular helix.

6. A broad-band resonator comprising a resonant cavity formed of a conductive casing, a tubular conductor extending into said casing through an opening in a wall of said casing and having its inner end insulated from said casing and being eifectively connected to said casing at the point of entry into said casing, a portion of said tubular conductor at the insulated end thereof being formed of a solid tubular section, the remaining portion of said tubular conductor being formed in a tubular helix, and means for producing relative movement between said wall of said casing and said conductor lengthwise of said conductor to vary the length of the portion of the tubular conductor which extends into said resonant cavity.

7. A broad-band oscillator comprising a resonant chamber formed of tubular outer and inner conductors, a short-circuiting slider arranged between said conductors and forming one end of the resonator cavity, said conductors being insulated from each other at the other end of the cavity, a portion of said center conductor at the said other end of said cavity being formed of a solid tubular section extending over a portion of the range of movement of said slider, and a portion of said center conductor extending over the low frequency section thereof being formed as a tubular helix; an electron tube having anode, cathode and grid elements, a connection from the insulated end of said inner conductor to said anode, a connection from said grid to said outer conductor having low impedance at high frequencies, an energizing circuit for heating said cathode, and a two-wire choke-coil included in said energizing circuit and having an antiresonant frequency near the lowest frequency of said resonant chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,203,085 Flechsig June 4, 1940 2,222,169 Buschbeck Nov. 19, 1940 2,252,370 Goldstine Aug. 12, 1941 2,428,272 Evans Sept. 30, 1947 2,431,179 Kentner Nov. 18, 1947 2,472,769 Goldstine June 7, 1949 2,516,887 Lehman Aug. 1, 1950 2,530,995 Rumpf Nov. 21, 1950 2,637,775 Lund May 5, 1953 2,641,708 Carlson June 9, 1953 2,645,718 Keizer July 14, 1953 2,663,799 Bell Dec. 22, 1953 FOREIGN PATENTS 476,906 Great Britain Dec. 17, 1937 OTHER REFERENCES Lund: Broadband Transition From Coaxial Line to Helix, R. C. A. Review, March 1950, volume XL, Issue 1, pp. 133-142. 

